How much grout for a mosaic tile backsplash?

The sheet size tells you nothing — the chip does

Daltile's Keystones mosaic spec sheet lists its 1 × 1 and its 2 × 2 on the same 12 × 24 in sheet, the same 1/4 in thickness and the same 24.00 sq ft per carton. Two products that are identical on paper, and one of them takes nearly twice the grout. At the 1/8 in joint Daltile recommends for the range, LATICRETE's coverage table gives 19 sq ft per 8 lb bag for the 1 in chip against 34 for the 2 in — 42.6 lb per 100 sq ft against 23.2. So the first thing to establish is not the size of the sheet in your hand but the size of one piece on it.

Penny rounds and hexes cost more than their nominal size

LATICRETE gives "1 in penny round" a row of its own, separate from 1 × 1 square: 27 sq ft per 8 lb bag at a 1/16 in joint where the square gets 34. A circle touches its neighbour at a single point, so the gap between chips is wider than the nominal joint almost everywhere, and you are filling the corners as well as the joints.

There is no round-chip input on this calculator, so use the substitution that matches the published figures: enter a 3/4 in square for a 1 in penny round. That returns 26.6, 14.8 and 10.9 sq ft per 8 lb bag at 1/16, 1/8 and 3/16 in joints, against LATICRETE's printed 27, 15 and 11.

The seam between sheets is a looks problem, not a quantity problem

Butting mesh sheets by eye leaves a run of wide joints every 12 in. Widening just that seam from 1/8 to 1/4 in adds about 6% to the grout on 1 in chips and 12% on 2 in chips — the bigger the chip, the fewer internal joints there are to dilute the seam. Both sit inside the 15% waste allowance, so it will not cost you a bag. Set a spacer in the seam anyway: the grid of wide lines is permanent and visible from the other side of the room.

Organic shapes have no single joint width

Daltile's installation note for its Organic Hex and Organic Penny mosaics says grout joints "are intended to vary due to the unique mosaic shape". With no fixed width there is no exact answer, so budget from the wide end — a 1 in organic penny priced at 3/16 in rather than 1/8 in goes from 15 sq ft per 8 lb bag to 11, which on a 30 sq ft backsplash is the difference between two bags and three.

Sources: Daltile Keystones ColorBody Porcelain Mosaic sales sheet (sizes, sheet sizes, thickness, sq ft per carton, grout joint recommendation) and LATICRETE PERMACOLOR Grout, DS-250 (approximate coverage, ft² per 8 lb bag).